1. Field of the Invention
The present invention relates to a thermal recording material and, more particularly, to a thermal recording material superior in color developing properties, color developing sensitivity (with a low level of fogging by heat), and durability (or resistance to plasticizer, oil, and water).
2. Description of the Related Art
There is known the thermal recording material which is based on a colorless or light-colored color former and a developer which causes the color former to develop a color upon heating. It is publicized in Japanese Patent Publication Nos. 4160/1968 and 14039/1970 and is in general practical use. The thermal recording material is usually composed of a support and a coating layer formed thereon. The support is paper, film, synthetic paper, or the like. The coating layer is formed from a solution in which are dispersed a leuco dye and a developer (of phenol derivative) in the form of fine particles. The solution also contains additives such as binding material, sensitizer, filler, and slip agent. Upon heating, either or both of the leuco dye and developer melt and come into contact with each other to bring about chemical reactions and develop a color for visible recording. The thermal recording material is prepared usually in the form of sheet. The color development of such thermal recording sheet is accomplished by means of a thermal printer provided with a thermal head. The thermal recording method has an advantage over other recording methods in (1) freedom from noise at the time of recording, (2) dispensability with development and fixing, (3) dispensability with maintenance, and (4) comparatively low machine price. Because of this advantage, it has gained wide acceptance in the fields of facsimile, computer output, calculator printer, medical instrument recorder, ticket vending machine, label printer, etc.
One of the major applications of the thermal recording is in the field of labels and price tags used in retail stores and supermarkets where the POS system has generally been accepted. However, thermosensitive labels attached to fresh produce and processed foods (such as box lunch) suffer the disadvantage of being discolored or becoming fogged. Discoloration (or disappearance of images) is due to plasticizer oozing out from the wrapping film or oils and fats leaking from the package. Fogging (or color development in the unprinted areas) is due to heating (for thawing) or humidity.
There is an increasing demand for a new thermal recording material to meet the requirement for high-speed recording. A problem to be solved for the development of such a new thermal recording material is that increased sensitivity (or improved thermal response) is offset by fogging. In addition, stabilizers to improve the resistance to plasticizer and oil are usually liable to decrease sensitivity and cause fogging. No satisfactory thermal recording materials have been completed yet which have improved thermal response, improved durability, and improved resistance to fog by heat and moisture.
Improvement of thermal response by supercalendering (to enhance smoothness) is mentioned in Japanese Patent Publication No. 20142/1977. Unfortunately, supercalendering needs complex operation and smooth surface is poor in compatibility with the thermal head. Also, improvement of thermal response by the use of a leuco dye having a low melting point is disclosed in Japanese Patent Laid-open No. 164890/1981. Unfortunately, improvement in this way is offset by increased heat fogging. There is another way of improving thermal response by interposing an intermediate layer between the support and the thermal recording layer, as disclosed in Japanese Patent Laid-open Nos. 5093/1984, 248390/1985, 113282/1989, and 214688/1990. The disadvantage of this method is the incomplete adhesion between the support and the thermal recording layer which leads to peeling by external force or under wet conditions.
Conventional thermal labels made of paper, synthetic paper, or polyester film pose a problem with their removal when containers or packaging materials of polystyrene or foamed polystyrene are recovered for recycling. To address this problem, using polystyrene film as the support for the thermal recording material has been proposed in Japanese Patent Laid-open No. 242061/1995. The disadvantage of this idea is that the color density is insufficient if the printing energy is low, which hampers the high-speed printing.